Hi Tvrtko, On Fri, Dec 02, 2022 at 10:20:11AM +0000, Tvrtko Ursulin wrote: > > On 01/12/2022 20:39, Andi Shyti wrote: > > From: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> > > > > Introduce the concept of padding the i915_vma with guard pages before > > and after. The major consequence is that all ordinary uses of i915_vma > > must use i915_vma_offset/i915_vma_size and not i915_vma.node.start/size > > directly, as the drm_mm_node will include the guard pages that surround > > our object. > > > > The biggest connundrum is how exactly to mix requesting a fixed address > > with guard pages, particularly through the existing uABI. The user does > > not know about guard pages, so such must be transparent to the user, and > > so the execobj.offset must be that of the object itself excluding the > > guard. So a PIN_OFFSET_FIXED must then be exclusive of the guard pages. > > The caveat is that some placements will be impossible with guard pages, > > as wrap arounds need to be avoided, and the vma itself will require a > > larger node. We must not report EINVAL but ENOSPC as these are unavailable > > locations within the GTT rather than conflicting user requirements. > > > > In the next patch, we start using guard pages for scanout objects. While > > these are limited to GGTT vma, on a few platforms these vma (or at least > > an alias of the vma) is shared with userspace, so we may leak the > > existence of such guards if we are not careful to ensure that the > > execobj.offset is transparent and excludes the guards. (On such platforms > > like ivb, without full-ppgtt, userspace has to use relocations so the > > presence of more untouchable regions within its GTT such be of no further > > issue.) > > > > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> > > Signed-off-by: Tejas Upadhyay <tejaskumarx.surendrakumar.upadhyay@xxxxxxxxx> > > Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx> > > Signed-off-by: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx> > > --- > > Hi Tvrtko, > > > > I removed your r-b in this version because I restored the original value > > of the guard being aligned with the vma size alignment. Turns out that > > CI failed with the latest version because the guard was becoming too big > > (we would have hit the GEM_BUG_ON)[*]. > > > > The reason why now the guard is aligned with the vma alignment is that > > the area is already aligned and if we use as a starting address start + > > guard, guard needs to be aligned, otherwise we screw up all the memory > > alignment. > > > > Let me know if it makes sense to you. > > Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx> > > Conditional to promise of a prioritised follow up improvement, if it turns > out GGTT wastage due a bit over zealous guard size comes to bite. Sure! I'll be alert! There are some unrelated failures from CI, just to be sure I sent last night a trybot run. Thanks, Tvrtko! Andi
